1. Technical Field of the Invention.
The present invention relates to a canine hip prosthesis, and more particularly to a femoral hip stem component and a porous coating for anchoring the stem in the intramedullary canal of the canine femur.
2. Description of Related Art.
Prior to the present invention a number of femoral inserts have been proposed for use in canine hip joint prostheses. Generally each of these included a polished spherical head or capitulum mounted on a stem, which stem is inserted into the intramedullary canal of the canine femur. Most often, the prior art inserts were cemented within the canal to hold them fast against the applied forces and loads. Some attempts have been made to anchor the insert without the use of cement by providing a mesh inset for the bone to grow into. These cementless inserts have not achieved optimum results.
It is well known in human medical practice to provide a metallic bone prosthesis with a porous metallic coating to enhance the fixation of the prosthesis to the patient's bone. Fixation of prostheses in humans by porous coatings, without the use of cement, has been found to eliminate several disadvantages associated with cemented fixation. In particular, these disadvantages include premature loosening of the prosthesis, tissue reaction with the cement and the need to remove large amounts of bone tissue to provide room for the cement. Tissue reaction and infection resulting from bone cement have been found to be more severe in dogs, making cementless fixation more desirable.
A variety of porous coatings have been proposed for human bone prostheses. For example, U.S. Pat. No. 3,855,638 discloses a porous coating for a surgical prosthetic device. However, this coating has been found unsuitable for use in human prosthetic devices because the small interstitial pore size and resulting porosity distribution tended to lead to failure of the bone at the coating-bone interface. For the same reason such a coating is unsuitable for canine applications. U.S. Pat. No. 4,550,448 discloses another porous coating for human bone protheses. This coating has been more successful due to its pore sizing and distribution, however, such a coating is unsuitable for use in canine bone prostheses. Use of the porous coating disclosed in the '448 patent in a canine femoral hip prosthesis would substantially degrade the strength of the prothesis due to the amount of stem material which would have to be removed to accommodate the coating thickness because the overall size of canine implant is significantly smaller than the human. This problem cannot be easily resolved by reducing the coating thickness because the thickness of the coating has been specifically designed for optimum bone ingrowth under human prosthetic conditions. To this point in time, a satisfactory porous coating for canine bone prostheses has not been developed.
Another problem in this field has been the overall shape of the insert. Prior art canine femoral hip prostheses have been provided with stems that exhibit a generally square cross-section and, except for an angled neck supporting the capitulum, exhibit substantially straight stems. Attempts have been made to computer model canine hip prostheses, however, these have proved less than satisfactory. One example of such a device has a wedge shaped stem which has exhibited a tendency to split the femur into which it is inserted.
The characteristics of the spacing or interface between the exterior surface of the prosthesis stem and the interior contour of the prepared cavity in the intramedullary canal play an important role in properly anchoring the insert to the femoral bone. A stem of sufficient length has long been recognized as desirable since it provides increased surface area for bone ingrowth within the canal and increased resistance to rotation. However, long stems require large intramedullary cavities, particularly at the calcar leading into the canal, since the overall surface thereof makes it difficult or impossible for the insert to pass into the canal without a significantly oversized entranceway. Such cavities result in large gaps or voids between the outside surface of the inserted stem and the inside surface of the cavity. Heretofore, a glove fit between a canine femoral stem and the intramedullary cavity together with superior anchoring of the stem and resistance to rotation have been considered difficult to achieve. Such a fit has been achieved in human femoral hip inserts (see, e.g. U.S. Pat. No. 4,589,883), but due to the different geometry of a human hip, this advance could not be used for canine applications.